We should use UserID object to identify users instead of username.

chrome/browser/ui/webui/chromeos/login/signin_screen_handler.h

Index: chrome/browser/ui/webui/chromeos/login/signin_screen_handler.h
diff --git a/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.h b/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.h
index 31df3b670526c70c76ccc11eb32d5ab000c790ab..650c30432df027a64533fa817ca1338aaca6ebf2 100644
--- a/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.h
+++ b/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.h
@@ -42,6 +42,10 @@ class DictionaryValue;
 class ListValue;
 }
 
+namespace user_manager {
+class UserID;
+} // namespace user_manager
+
 namespace chromeos {
 
 class CaptivePortalWindowProxy;
@@ -77,7 +81,7 @@ class LoginDisplayWebUIHandler {
  public:
   virtual void ClearAndEnablePassword() = 0;
   virtual void ClearUserPodPassword() = 0;
-  virtual void OnUserRemoved(const std::string& username) = 0;
+  virtual void OnUserRemoved(const user_manager::UserID& user_id) = 0;
   virtual void OnUserImageChanged(const user_manager::User& user) = 0;
   virtual void OnPreferencesChanged() = 0;
   virtual void ResetSigninScreenHandlerDelegate() = 0;
@@ -86,12 +90,12 @@ class LoginDisplayWebUIHandler {
                          const std::string& help_link_text,
                          HelpAppLauncher::HelpTopic help_topic_id) = 0;
   virtual void ShowErrorScreen(LoginDisplay::SigninError error_id) = 0;
-  virtual void ShowGaiaPasswordChanged(const std::string& username) = 0;
-  virtual void ShowSigninUI(const std::string& email) = 0;
+  virtual void ShowGaiaPasswordChanged(const user_manager::UserID& user_id) = 0;
+  virtual void ShowSigninUI(const user_manager::UserID& user_id) = 0;
   virtual void ShowPasswordChangedDialog(bool show_password_error,
-                                         const std::string& email) = 0;
+                                         const user_manager::UserID& user_id) = 0;
   // Show sign-in screen for the given credentials.
-  virtual void ShowSigninScreenForCreds(const std::string& username,
+  virtual void ShowSigninScreenForCreds(const user_manager::UserID& user_id,
                                         const std::string& password) = 0;
   virtual void ShowWhitelistCheckFailedError() = 0;
   virtual void LoadUsers(const base::ListValue& users_list,
@@ -163,13 +167,13 @@ class SigninScreenHandlerDelegate {
   virtual void CancelUserAdding() = 0;
 
   // Load wallpaper for given |username|.
-  virtual void LoadWallpaper(const std::string& username) = 0;
+  virtual void LoadWallpaper(const user_manager::UserID& user_id) = 0;
 
   // Loads the default sign-in wallpaper.
   virtual void LoadSigninWallpaper() = 0;
 
   // Attempts to remove given user.
-  virtual void RemoveUser(const std::string& username) = 0;
+  virtual void RemoveUser(const user_manager::UserID& user_id) = 0;
 
   // Let the delegate know about the handler it is supposed to be using.
   virtual void SetWebUIHandler(LoginDisplayWebUIHandler* webui_handler) = 0;
@@ -191,7 +195,7 @@ class SigninScreenHandlerDelegate {
   virtual void HandleGetUsers() = 0;
 
   // Runs an OAuth token validation check for user.
-  virtual void CheckUserStatus(const std::string& user_id) = 0;
+  virtual void CheckUserStatus(const user_manager::UserID& user_id) = 0;
 
  protected:
   virtual ~SigninScreenHandlerDelegate() {}
@@ -215,12 +219,12 @@ class SigninScreenHandler
       GaiaScreenHandler* gaia_screen_handler);
   ~SigninScreenHandler() override;
 
-  static std::string GetUserLRUInputMethod(const std::string& username);
+  static std::string GetUserLRUInputMethod(const user_manager::UserID& user_id);
 
   // Update current input method (namely keyboard layout) in the given IME state
   // to LRU by this user.
   static void SetUserInputMethod(
-      const std::string& username,
+      const user_manager::UserID& user_id,
       input_method::InputMethodManager::State* ime_state);
 
   // Shows the sign in screen.
@@ -288,7 +292,7 @@ class SigninScreenHandler
   // LoginDisplayWebUIHandler implementation:
   void ClearAndEnablePassword() override;
   void ClearUserPodPassword() override;
-  void OnUserRemoved(const std::string& username) override;
+  void OnUserRemoved(const user_manager::UserID& user_id) override;
   void OnUserImageChanged(const user_manager::User& user) override;
   void OnPreferencesChanged() override;
   void ResetSigninScreenHandlerDelegate() override;
@@ -296,12 +300,12 @@ class SigninScreenHandler
                  const std::string& error_text,
                  const std::string& help_link_text,
                  HelpAppLauncher::HelpTopic help_topic_id) override;
-  void ShowGaiaPasswordChanged(const std::string& username) override;
-  void ShowSigninUI(const std::string& email) override;
+  void ShowGaiaPasswordChanged(const user_manager::UserID& user_id) override;
+  void ShowSigninUI(const user_manager::UserID& user_id) override;
   void ShowPasswordChangedDialog(bool show_password_error,
-                                 const std::string& email) override;
+                                 const user_manager::UserID& user_id) override;
   void ShowErrorScreen(LoginDisplay::SigninError error_id) override;
-  void ShowSigninScreenForCreds(const std::string& username,
+  void ShowSigninScreenForCreds(const user_manager::UserID& user_id,
                                 const std::string& password) override;
   void ShowWhitelistCheckFailedError() override;
   void LoadUsers(const base::ListValue& users_list, bool show_guest) override;
@@ -373,7 +377,7 @@ class SigninScreenHandler
   // Sends the list of |keyboard_layouts| available for the |locale| that is
   // currently selected for the public session identified by |user_id|.
   void SendPublicSessionKeyboardLayouts(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       const std::string& locale,
       scoped_ptr<base::ListValue> keyboard_layouts);
 
@@ -422,7 +426,7 @@ class SigninScreenHandler
   // Gets the easy unlock service associated with the user. Can return NULL if
   // user cannot be found, or there is not associated service.
   EasyUnlockService* GetEasyUnlockServiceForUser(
-      const std::string& username) const;
+      const user_manager::UserID& user_id) const;
 
   // Current UI state of the signin screen.
   UIState ui_state_ = UI_STATE_UNKNOWN;